(a) Field of the Invention
The present invention relates to an acrylic pressure-sensitive adhesive composition for a polarizing film, more particularly to a pressure-sensitive adhesive composition for a polarizing film capable of solving the problem of light leakage, a polarizing film prepared using the same and a liquid crystal display employing the same.
(b) Description of the Related Art
Basically, a liquid crystal cell containing liquid crystal, a polarizing film and an adhesive layer or a pressure-sensitive adhesive layer for joining the two are required to manufacture a liquid crystal display. In addition, a phase retardation film, a compensation film for wide view angle, a brightness enhancement film, etc. may be further attached to the polarizing film to improve performance of the liquid crystal display.
Typically, a liquid crystal display comprises a uniformly aligned liquid crystal layer; a polarizing film comprising a liquid crystal cell, which consists of a transparent glass plate or a plastic plate including a transparent electrode layer and an adhesive layer or a pressure-sensitive adhesive layer; a phase retardation film; and additional functional film layers.
The polarizing film consists of a uniformly aligned iodine compound or dichroic polarizing material. To protect these polarizing elements, a protection film made of triacetyl cellulose (TAC), etc. is used. The polarizing film may further comprise a phase retardation film having an anisotropic molecular alignment, a compensation film for wide view angle such as an optically designed liquid crystal film, etc.
Because the aforementioned films are made of materials having different molecular structures and compositions, they have different physical properties. Especially, under a specific thermal and/or humidity condition, materials having an anisotropic molecular alignment shrink or expand, which causes the lack of dimensional stability. As a result, if the polarizing film is fixed by a pressure-sensitive adhesive, a shear stress caused the thermal and/or humidity condition remains, so that light leakage occurs at the region where the stress is concentrated.
One way to solve the problem of light leakage is to reduce the shrinkage of the polarizing film at the thermal and/or humidity condition. However, it is very difficult to remove the stress applied to a liquid crystal panel to which a polarizing film consisting of different materials has been attached.
Rubbers, acryls and silicones are commonly used as pressure-sensitive adhesives. Among these, acrylic pressure-sensitive adhesives are used the most widely in manufacturing high performance pressure-sensitive adhesive compositions.
However, when a polarizing film manufactured using such a pressure-sensitive adhesive is used for a long time (under a specific thermal and/or humidity condition) in a liquid crystal display, stress tends to concentrate because of the shrinkage of the polarizing film. To solve this problem, the adhesive layer needs to have a stress relaxation ability.
U.S. Pat. No. 5,795,650 offered such ability to a pressure-sensitive adhesive by adding a plasticizer to the adhesive layer. However, the light leakage problem could not be solved and it was known that the plasticizer significantly impairs adhesion property of the adhesive because of its surface transfer characteristic. As a result, the adhesive does not solve such durability and reliability problems as bubble and edge lifting.
Japan Patent Publication No. Hei 10-279907 proposed a method of solving the light leakage problem by offering a stress relaxation ability, which was achieved by mixing an acrylic polymer having a high molecular weight with an acrylic polymer having a low molecular weight of 30,000 or less. However, the decrease of the light leakage is slight and the adhesion durability and reliability are reduced significantly because of the surface transfer characteristic. In addition, the cutting property worsens.
It is also possible to control crosslinking density with a chemical bonding, in order to offer stress relaxation ability to the adhesive. But, this method is also insufficient to solve stress concentration at the polarizing film. Besides, if the crosslinking density is too low, the durability may worsen. There is an alternative of offering fluidity to the adhesive by adding a plasticizer or a material having a low molecular weight. But, the added materials may significantly worsen the adhesive performance because of their surface transfer characteristic.
Thus, the development of a new adhesive for a polarizing film capable of solving the light leakage problem without compromising such major characteristics as adhesion durability and cuttability and a polarizing film using the same are urgently needed.